It has been recognized that hot weather can cause problems in working with concrete and that such problems increase as temperatures rise. For example, it has been reported that it may be necessary to make adjustments to a concrete mix as the weather warms because an everyday mix can begin to perform differently as temperatures rise above about 23° C. (75° F.). It has also been recognized that hot weather is a combination of the following weather conditions: High ambient temperature; High concrete temperature; Low relative humidity; Solar radiation; and High wind speed.
In the Middle East and some parts of the United States, ambient temperatures of about 32° to 45° C. are frequently encountered during summer months. For example, in 1913 in Death Valley, Calif. a temperature of 134° F. (56.6° C.) was recorded. Problems associated with extreme heat i.e. 32° C. (90°) to 40 or 45° C. (104-113° F.) or greater include increased water demand, increased rate of slump loss, increased rate of setting, increased tendency for plastic-shrinkage cracking and increased difficulty in controlling entrained air content.
The main problem affecting hardened concrete is the decreased long-term strength.
Authorities in concrete technology such as individuals in the American Concrete Institute (ACI) and Portland Cement Association (PCA) recommend cooling concrete as low as possible in order to obtain good quality concrete in hot weather. In addition, some specifications require that when placed, concrete should have a temperature of less than 29° C. (84° F.) to 32° C. (90° F.). Therefore, in areas of extreme temperature, ice or chilled water is added to a hydraulic cement, aggregate and water mix. Further, in adding ice, it is important to use crushed, shaved or chipped ice to ensure all of the ice melts before mixing is completed. Reducing temperatures of a concrete mix adds substantially to the cost of the process.
Set retarding/water-reducing admixtures are sometimes used to counteract many of the negative impacts of hot weather on fresh concrete, particularly the rapid setting caused by high temperatures. Water-reducing admixtures can help curb slump loss without affecting the water demand of the mix. However, chemical admixtures are conditional on cement type and require care in adjusting dosage. Admixtures that increase the bleeding rate can help counteract surface drying, but may also require additional consolidation after the majority of bleeding has subsided.